In association with rapid popularization of the Internet in recent years, a transfer capacity of a backbone optical network provided by a service provider has been becoming increasingly larger. Therefore, there is the strong need for development of a large capacity, low cost, and low power consumption optical module for connected in or between the devices in a facility where a large capacity router, a multiplex transmission device or the like are installed.
There have been proposed various ideas for the optical modules satisfying the need as described above, and there is known the system in which a surface emitting laser represented by the VCSEL (Vertical Cavity Surface Emitting Laser) is used as a light source because of the low cost and the lower power consumption, and also in which photoelectric conversion in the light-receiving side is performed by a surface photodetector represented by the PIN photodiode. Furthermore, a multi-channel optical module having a large capacity is often used in which a plurality of photonic devices is arrayed with a pitch of 250 μm to make use of the low power consumption characteristics of the VCSEL.
In the optical module as described above, a micro lens array is used for realizing tight optical coupling between arrayed photonic devices and an optical fiber array. To prevent generation of stray light between adjoining channels, a diameter of each lens in the micro lens array is at most 250 μm which is identical to a channel pitch between adjoining photonic devices. Therefore, when a spreading angle of outgoing light from a surface emitting laser or an optical fiber is taken into considerations, a distance between a photonic device and a lens and a distance between a lens and an optical fiber are several hundreds μm respectively.
In the optical module in which optical components are provided at positions close to each other as described above, it is necessary to precisely adjust alignment between a micro lens array and photonic devices. For instance, a case is examined below in which a laser with a spreading half angle of outgoing light from a surface emitting laser of about 12 degrees and a multi-mode fiber with a core diameter of 50 μm are optically connected with a convex lens with a lens diameter of 250 μm and a curvature radius of 170 μm. In this case, a direction along the optical axis is defined as axis Z, and directions perpendicular to the optical axis as axis X and axis Y. When a position of a photonic device is displaced by +70 μm in the Z-axial direction and by +10 μm in the X- and Y-axial directions from a relative position of optical components where the maximum coupling efficiency is realized, the coupling efficiency is degraded by 1 dB or more. This fact suggests that highly precise alignment between the photonic device and the lens is required especially in the X- and Y-axial directions.
Recently, in the multi-channel optical module, a data transfer rate per channel of 10 Gbit/s or more is required to satisfy the requirement for a larger capacity. Thus further secure optical coupling is required, and specifically to realize precise alignment between a photonic device and a lens in the X- and Y-axial directions, position adjustment is required with several μm.
Various ideas have been proposed for a method of mounting a photonic device and a lens. One of the methods is disclosed, for instance, in Japanese Patent Laid-Open Publication No. 2001-116962. In the method disclosed in this document, at first a photonic device is mounted on a board for a photonic device, and an end face of the board and a lens array block are engaged with each other by engagement between a concave portion and a convex portion, or between a trench-like concave portion and a column-like protrusion or a column-like member for position adjustment. In this method, however, all of a precision in manufacturing an engagement section for a substrate, a precision in mounting a photonic device onto a board for a photonic device, and a precision in manufacturing an engagement section for a lens carrier contribute to positional displacement between the photonic device and the lens. In this case it is substantially impossible to limit the positional displacement within several μm. Especially it is not practical, from the view point of mass productivity and cost, to manufacture an engagement section for a board for a photonic device with high precision by machining. Therefore, it can be understood that the manufacturing method as described above is not adapted to manufacture of a high speed optical module ensuring strong optical coupling currently required. On the other hand, Japanese Patent Laid-Open Publication No. 2001-88148 discloses a method in which a lens and a photonic device are mounted without machining or the like. In this method, alignment marks are provided on a glass substrate covering a liquid crystal panel and a lens carrier, and position adjustment for the two optical members is performed by visually checking and adjusting the alignment marks.
FIG. 1 is a view schematically showing a case in which position alignment with alignment marks like in the prior art is applied to an optical module. A surface emitting laser 106 and an IC 108 are mounted on a board for a photonic device 110, and also a pedestal 801 is mounted thereon. A lens 105 is provided at a position opposite to the board for a photonic device 110, and also a lens member 101 supported by the pedestal 801, is provided thereon. In this step, position adjustment in the X- and Y-axial directions is performed by visually checking from a top position in the Z-axial direction and adjusting relative positions of an alignment mark 104a provided on a surface of the lens ember 101 and an alignment mark 104b provided on the surface emitting laser 106. A position of the lens member 101 in the Z-axial direction can be decided according to a height of the pedestal 801. Reference numeral 107 denote wiring for connection between the surface emitting laser 106 to the IC 108, and reference numeral 109 denote wiring for connection between the IC 108 and an external circuit. This method provides the advantage that, because the alignment mark 104a provided on the lens member 101 and the alignment mark 104b provided on the surface emitting laser 106 can be manufactured by casting or by a semiconductor process, the mark positions are precise and the mass productivity is excellent. On the other hand, the method has the defects as described below.
Descriptions are provided for a case where the currently most standard 12-channel surface emitting laser 106 and the micro lens array 105 are aligned by means of image recognition with an optical device such as a microscope. In this case, even when the view field of the lens is taken into consideration, a numerical aperture (NA) of the microscope is preferably in the range from about 0.05 to about 0.3. A resolution of the microscope is in inverse proportion to a numerical aperture (NA) of the optical system, and the focal depth in inverse proportion to a square of the numerical aperture (NA). For instance, a microscope having a diameter of a lens view field of 4.5 mm and a numerical aperture of 0.23 has a lens resolution of 1.5 μm and a focal depth of around +5 μm. In this case, even when a distance between an alignment mark 104a on the lens member 101 and an alignment mark 104b on the surface emitting laser 106 are apart from each other by 10 μm, the position adjustment can be performed with a precision of about 1.5 μm at maximum. In a case of a microscope having the numerical aperture of 0.06, the lens resolution is 5.5 μm and the focal depth is about +80 μm. It is generally known that the practical precision of position adjustment is a little lower than the lens resolution. Therefore, when a photonic device and a lens are aligned within the precision of +10 μm by using alignment marks, a light path length between the two marks is preferably at most 200 μm.
However, because it is industrially impossible to manufacture an micro lens array having a small curvature radius (1), and also because there is an electrode pad on a top surface of the surface emitting laser 106 and wire is bonded to the pad so that the lens member 101 cannot directly be accessed (2), the lens member 101 and the surface emitting laser 106 are apart from each other by about 200 μm to about 500 μm in terms of a light path length. With the structure based on the prior art, also a distance between the two marks is larger, so that it is impossible to align the photonic device and the lens with high positional precision.
Patent document 1: Japanese Patent Laid-Open Publication No. 2001-116962
Patent document 2: Japanese Patent Laid-Open Publication No. 2001-88148